Abstract – 15
Review paper on permeable pavement systems
Kolluru Hemanth Kumar
Sch.no - 121111049, B.tech 5rd sem, Department of civil engineering, Maulana Azad National Institute of Technology, Bhopal, India.
Abstract
The purpose of this review paper is to compact the wide range and spread the relevant works on permeable pavement systems, and also to deal with the current research and line of works to recommend the future areas of research work, also outlined about the important role of the permeable pavement systems in sustainable urban drainage systems in both the traditional and modern context, and discussed in brief about the permeable pavement systems. Particular emphasise is given for water quality control aspects which includes the microbiological point of view of permeable pavement system and the pollutants. Recent research on the combined geothermal heating and cooling, water treatment and recycling pavement system is promising which are discussed in short. At last the future research and innovations are discussed in briefly.
The intent of this review is to provide stakeholders in storm water management with the critical information that is needed for the foster acceptance of permeable pavement systems as a viable alternative to the traditional systems.
Keywords: permeable pavement system, sustainable urban development system, geothermal heating and cooling system, recycling pavement system, porous pavement, storm water management.
1. Introduction
1.1 Sustainable urban drainage systems
Sustainable Urban Drainage Systems has many other names like sustainable drainage schemes and sustainable drainage systems. Whatever: the key point is that it is sustainable drainage – a series of methods for dealing with drainage in a sustainable manner, using technologies that minimise the environmental impact of our lifestyles, buildings, structures and surfaces. Sustainable Urban Drainage Systems challenge the traditional approach of wastewater treatment by optimising the resource utilisation and development of novel and more productive technologies. There are numerous advantages by implementing Sustainable Urban Drainage Systems in our society but a few key pluses are mentioned below. Which are:
Flood control and better management of storm water at source (source control) Pollution control
Recharging of groundwater regimes and aquifers Reduced construction and maintenance costs Improved environment.
1.2 Technologies of SUDS
There are dozens, if not hundreds of different SUDS applications, ranging from reed-bed treatment systems for polluted water, to settlement ponds for sediment, to simple swales and filter drains. Schemes are usually site- specific, taking a range of core technologies and using them either singly or in combination to create and application that deals with the surface water drainage for a particular site. One of such technology is Permeable
Figure. 1. Difference between Permeable Paving and SUDS
Paving or Permeable Pavement Systems, but there is a subtle difference between these two which is permeable pavements allow water to pass through the paving structure, whereas suds-friendly pavements simply direct surface water to a suds installation such as a soakaway, a swale, etc. The permeable pavement systems and SUDS are differentiated in the Figure. 1. Suds-compliant pavements which can be defined as any pavement from which surface water is sent to a suds installation from where it may have the opportunity to drain to ground or be
temporarily stored rather than being directly channelled into the public sewer system or an open watercourse. 2. Permeable pavement systems
Permeable Pavement Systems are designed to achieve water quality and quantity benefits by allowing movement of storm water through the pavement surface and into a base/sub base reservoir, the water passes through the voids in the pavement materials (or) through the gap between pavers and provides the structural support as conventional
pavement. That’s why permeable pavements can be served as an alternative to conventional road and parking lots. These pavements provides the ability to reduce urban runoff and also provide the opportunities to mitigate the impacts of urbanization on receiving water systems by providing at source treatment and management of storm water. Permeable pavement systems have been shown to improve the storm water quality by reducing the storm water temperature, pollutant concentrations and pollutant loading of suspended solids, heavy metals, polyaromatic hydrocarbons and some nutrients. Purposes for using permeable pavement systems instead of using other pavement systems are mentioned as:
Promote storm water infiltration, groundwater recharge, and stream base flow preservation 1. Reduce the discharge of storm water pollutants to surface waters
2. Reduce storm water discharge volumes and rates 3. Reduce the temperature of storm water discharges
2.1 Types of Permeable Pavement Systems
There are generally permeable varieties of asphalt, concrete, and interlocking pavers that means depending upon the type of materials used permeable pavement systems are divided into various types. Which are:
Figure. 2. Figure showing different types of permeable pavement systems
2.2.1 Permeable Asphalt
Permeable asphalt, also known as pervious, porous, "popcorn," or open-graded asphalt, is standard hot-mix asphalt with reduced sand or fines and allows water to drain through it. It generally consists of fine and coarse aggregate stone bound by a bituminous-based binder coarse. Porous asphalt over an aggregate storage bed will reduce storm water runoff volume, rate, and pollutants. The reduced fines leave stable air pockets in the asphalt. The interconnected void space allows storm water to flow through the asphalt as shown in Figure. 2., and enter a crushed stone aggregate bedding layer and base that supports the asphalt while providing storage and runoff treatment. When properly constructed, porous asphalt is a durable and cost competitive alternative to conventional asphalt. The void space can be increased typically up to 15 to 20 % by reducing the amount of fine aggregate. Thickness of the asphalt depends on the traffic load, generally ranges from 7.5 to 18 cm (3-7 in). By introducing underlying base course increases storage and adds strength to the pavement. There were problems in the past with early porous asphalt, as the binder would migrate into the higher void spaces, blocking the travel path of the water. This has been ameliorated with the use of additives and additional binders. (NCDENR, 2007; Hun-Dorris, 2005) Additives and additional binders are often used to enhance the characteristics of porous asphalt. Polymers keep the binder from migrating into the void spaces. Polymer-reinforcing fibers assist with cohesion of the mix. (Hun-Dorris, 2005). Porous asphalt can be used for municipal storm water management programs and private development applications. The runoff volume and rate control, pollutant reductions allow municipalities to improve the quality of storm water discharges. The use of porous asphalt can potentially reduce additional expenditures and land consumption for conventional collection, conveyance, and detention storm water infrastructure. Porous asphalt can replace traditional impervious pavement for most pedestrian and vehicular applications. Porous asphalt performs well in pedestrian walkways, sidewalks, driveways, parking lots, and low-volume roadways. The environmental benefits from porous asphalt allow it to be incorporated into municipal
green infrastructure and low impact development programs. Open-graded asphalt has been used for decades as a friction course over impervious asphalt on highways to reduce noise, spray, and skidding.
2.2.2 Permeable concrete
Permeable concrete, also known as pervious (or) porous concrete, gap-graded (or) enhanced porosity concrete, is concrete with reduced sand or fines and allows water to drain through it. . Pervious concrete pavement is a unique and effective means to address important environmental issues and support green, sustainable growth by capturing storm water and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing storm water runoff. Pervious concrete is also known as no-fines or low fines concrete, it is a mix of Portland cement, coarse aggregate, water and admixtures. Because there is little or no sand in the mix, the pore structure contains many voids that allow water and air to pass through. A typical pervious concrete pavement has a 15-25% void structure and allows 3-8 gallons of water per minute to pass through each square foot. Carefully controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles without flowing off during mixing and placing. Using just enough paste to coat the particles maintains a system of interconnected voids. The result is a very high permeability concrete that drains quickly. Due to the high void content, pervious concrete is also lightweight, 1600 to 1900 kg/m3 (100 to 120 lb. /ft3). After placement or after paved, pervious concrete resembles popcorn as shown in Figure. 2. Its low paste content and low fine aggregate content make the mixture harsh, with a very low slump. The compressive strength of pervious concrete is limited since the void content is so high. However, compressive strengths of 3.5 to 27.5 MPa (500 psi to 4000 psi) are typical and sufficient for many applications such as Parking lots, Driveways, Sidewalks/walkways, Streets/road shoulders other light traffic areas. Pervious concrete is a durable material; properly designed and constructed streets and parking lots will last many years with minimal maintenance. Thus concrete, conventional or pervious, is widely recognized as the lowest life cycle cost option available for pavement material selection. Pervious concrete also contributes to enhanced air quality by lowering atmospheric heating through light colour and low density, decreasing the heat island effect which occurs with dark pavement surfaces. The heat island effect is characterized by an up to 4° C average temperature increase between an urban area and its surrounding countryside and an increase in the probability of smog due to the higher temperatures. (Gajda, J.W., Van Geem, M.G., 1997).
2.2.3 Permeable interlocking concrete pavement
Permeable interlocking concrete pavement (PICP) consists of manufactured concrete units that reduce storm water runoff volume, rate, and pollutants. The impervious units are designed with small openings between permeable joints. The openings typically comprise 5% to 15% of the paver surface area and are filled with highly permeable, small-sized aggregates. The joints allow storm water to enter a crushed stone aggregate bedding layer and "open- graded" base i.e., crushed stone layers with no small or fine particles. That supports the pavers while providing storage and runoff treatment. The void spaces among the crushed stones store water and infiltrate it back into the soil subgrade. The stones in the joints provide 100% surface permeability and the base filters storm water and reduces pollutants. PICPs are highly attractive, durable, easily repaired, require low maintenance, and can withstand heavy vehicle loads. Pervious concrete and porous asphalt rely on small- sized aggregates bound with asphalt or cement to create a porous matrix that supports vehicular traffic. In contrast, PICP relies on solid, high-strength concrete units to support traffic surrounded by small, highly pervious stone-filled joints to receive and infiltrate storm water. The stone-filled joints also contribute to interlocking and spreading wheel loads. Depending on the paving unit design and pattern, PICP joints can vary between 1/8 and 1/2 in. (3 and 13 mm). Small-sized aggregate in the joints that allow water to pass through it can be somewhat deceptive. While PICP has less visible permeable surface area than porous asphalt or pervious concrete, PICP openings still provide high surface infiltration rates. These rates are well above practically all rainfall intensities, making their hydrological performance equal to or better than other permeable surfaces. The small aggregate in the joints and bedding also facilitates interlock and load transfer to neighbouring pavers. Unlike standard interlocking concrete pavement, no sand is used in PICP joints or bedding since it has very low permeability. The pavers are available in many different shapes and sizes. Paving surface can be of different types depending upon the type of pavers used. They are: interlocking shapes with openings, enlarged permeable joints, and Porous concrete. ASTM C936 specifications (200 lb) state that the pavers can be of size 60 mm (2.36 in) thick with a compressive strength of 55 Mpa or greater. Typical installations consist of the pavers and gravel fill, a 38 to 76 mm (1.5-3.0 in) fine gravel bedding layer, and a gravel base storage layer is constructed. PICP should not be confused with concrete grid pavements (i.e., concrete units with cells that typically contain topsoil and grass). Concrete grid pavement paving units can also infiltrate water, but at rates lower than PICP. Unlike PICP, concrete grid pavements are generally not designed with an open-graded, crushed stone base for water storage. Moreover, grids are for intermittently trafficked areas such as overflow parking areas and emergency fire lanes.
2.2.4 Concrete grid pavers
Concrete grid pavements “green parking lots” provide a cool, green surface as shown in (Figure. 2.) Solution for vehicular access lanes, emergency access areas, and overflow parking areas, and even residential driveways. Grids are proven contributors to reduced ambient urban temperatures thereby contributing to reduced heat island while
taking in some rainfall and runoff. Perforated concrete units as pavement were introduced when hollow concrete building blocks were placed in the ground to support cars. The properties of concrete grid units are defined in ASTM C 1319, Standard Specification for Concrete Grid Paving Units (ASTM C, 1319-95). This specification defines concrete grids as having maximum dimensions of 24 in. long by 24 in. wide (610 mm by 610 mm) and a minimum nominal thickness of 31/8 in. (80 mm). The minimum required thickness of the webs between the openings is 1 in. (25 mm). The percentage of open area ranges from 20 to 50 % and can contain topsoil and grass, sand, or aggregates in void spaces. A typical installation consists of grid pavers with fill media, 25 to 38 mm (1-1.5 in) of bedding sand, gravel base course, and a compacted soil subgrade (ICPI, 2004) Dimensional tolerances should not differ from approved samples more than 1/8 in. (3.2 mm) for length, width, and height. The minimum compressive strength of the concrete grid units should average 5,000 psi (35 MPa) with no individual one less than 4,500 psi (31 Mpa). Concrete grid pavers range in weight from 45 lbs. (20 kg) to 90 lbs. (40 kg). Concrete grid unit designs fall into two categories: Lattice, and Castellated.
Lattice pavers have a flat surface that forms a continuous pattern of concrete when installed. Castellated grids include protruding concrete knobs on the surface making the grass appear continuous when installed.
2.2.5 Plastic Reinforcement Grid Pavers
Plastic reinforcement grid pavers also called geocells, consists of flexible plastic interlocking units that allow for infiltration through large gaps filled with gravel or topsoil planted with turf grass. A sand bedding layer and gravel base-course are often added to increase infiltration and storage. The empty grids are typically 90 to 98 percent open space, so void space depends on the fill media (Ferguson, 2005). To date, no uniform standards exist; however, one product specification defines the typical load- bearing capacity of empty grids at approximately 13.8 MPa (2,000 psi). This value increases up to 38 MPa (5,500 psi) when filled with various materials (Invisible Structures, 2001). Generally the grid need to be installed onto a geogrid or geotextile covered by a layer of gravel bedding. The grids pavers can be filled with soil and grassed as the octagonal honeycomb cell structure as shown in the Figure. 8. and open base promotes unrestricted root growth, or can be filled with gravel giving a high quality decorative look. The hexagonal cell structure will retain the gravel and prevent loss or gravel displacement. The paving system is the perfect solution for car parking for on grass that constantly becomes worn, muddy (or) rutted. Perfect for lawn reinforcement, farm gateway entrances, grass roads, grass access routes and golf buggy paths. 3. Water quality
3.1. Pollutants
Pollution which presents on the road and car park surfaces as a result of oil and fuel leaks, and drips, tyre wear, and dust from the atmosphere. This type of pollution arises from a wide variety of sources and is spread throughout an urban area also known as diffuse pollution. Rainwater washes the pollutants off the surfaces. Conventional drainage systems, as well as attenuation tanks, effectively concentrate pollutants, which are flushed directly into the drainage system during rainfall and then into watercourses or groundwater. The impact of this is to reduce the environmental quality of watercourses. Impervious surfaces have a high potential for introducing pollution to watercourses. Possible water quality variables of concern include the following as stated in D’Arcy et al. (1998) and NCDENR (2005):
• Sediment and suspended solids (including phosphorus and some metals); • Organic waste with high biochemical oxygen demand;
• Oil and grease; and faecal pathogens.
• Dissolved nutrients and pollutants (including nitrogen, heavy metals, solvents, herbicides and pesticides);
Five processes that affect the concentration of pollutants in the soil’s unsaturated zone: Sorption, Filtration, Degradation, Volatilization, and Water transport. Sorption tends to be the dominant process, and most pollutants will either be sorbed to soil particles or to organic matter. Storm water pollutants can also be adsorbed onto surrounding sediments in the storm water before they infiltrate into the soil. (Mikkelsen et al. (1994)) compiled research data showing that approximately 50 percent of the heavy metal load in storm water could be removed through either sedimentation or filtration. Permeable pavements have a good track record at removing suspended solids and nitrogen. However, PPS, which do not rely on below ground infiltration and the use of an underdrain system, will not be successful in the removal of nitrogen. When an underdrain system is incorporated into the pavement design, storm water tends not to infiltrate into the soil, but into the underdrain, where it can be denitrified or removed by plant uptake (NCDENR, 2005). (Dierkes et al. (2002)) summarised possible ranges of pollutant concentrations in rain, and roof and road runoff, taken from more than 60 sites throughout Europe. Rain may contain 5-day biochemical oxygen demand (1–2 mg/l), sulphate (0.56–14.40 mg/l), chloride (0.2–5.2 mg/l), ammonia (0.1–2.0 mg/l), nitrate (0.l–7.4 mg/l), and total phosphate (0.01–0.19 mg/l), copper (1–355 mg/l) and zinc (5–235 mg/l). Another laboratory study by Fach and Geiger, (2005) examined pollution removal rates of Cd, Zn, Pb, and Cu for permeable concrete block paving and three variations of concrete block pavers; one with wide infiltration pores (29mm), another with narrow pores (3mm), and pavers with a crushed brick substrate infill. Permeable concrete have the highest heavy metal removal rate (96.5% average) followed by block paving with
brick substrate infill (92.9% average). No significant differences between the narrow infiltration pores and the wide pores were observed (63.1% and 78.6% average removal rates, respectively). When set over a 4 cm crushed basalt or brick substrate roadbed and a 40 cm limestone base course, average pollution removal rates for all pavements and substructures were very high, ranging from 96 to 99.8 percent.
3.2. Hydrocarbons
Oil and diesel fuel contamination is frequently detected on asphalt and other non-permeable surfaces. In comparison, these contaminants were not detected on PPS surfaces assessed by Bratterbo and Booth. Hydrocarbons can endanger soil and groundwater, if they are not removed sufficiently during infiltration through the surface layer. Many pollutants such as polycyclic aromatic hydrocarbons, metals, phosphorous and organic compounds are absorbed onto suspended solids. Models have been designed to estimate the suspended solids load and its dynamics during rainfall events, leading to better understanding of receiving waters being polluted by hydrocarbons. Concerning various pavement systems, Booth showed that infiltrated water had significantly lower levels of copper and zinc in comparison to the direct surface runoff from the asphalt area. Motor oil was detected in 89% of samples from the asphalt runoff, but not in any outflow water sample from the PPS. Diesel fuel was not detected in any sample. Infiltrate measured five years earlier displayed significantly higher concentrations of zinc, and significantly lower concentrations of copper and lead. Permeable pavements can operate as efficient hydrocarbon traps and powerful in-situ bioreactors. Coupe et al. found out that a PPS specifically inoculated with hydrocarbon- degrading microorganisms does not successfully retain a viable population of organisms for the purpose of increased hydrocarbon degradation over many years. Naturally developed microbial communities (i.e. no inoculation with allochthonous microorganisms) degrade oil successfully. For the successful biodegradation of polycyclic aromatic hydrocarbons, certain environmental conditions need to be met. Degradation takes place when prolonged aerobic, sulphate reducing and denitrifying conditions occur. Very large hydrocarbon spills can be contained due to absorption processes within the pavement. Wilson incorporated an oil interceptor into a porous surface construction. Tests were carried out for worst-case scenarios such as the worst possible combined pollution and rainfall event to assess how the system retains pollutants within its structure. The results successfully demonstrated that this system can retain hydrocarbons, and can therefore offer outflow with improved water quality. However, where certain detergents are present in the pavement system, they can cause contamination of the outflow water, which may require secondary treatment to improve its water quality.
3.3. Metals
Studies have shown an improvement of water quality by filtration through PPS, which work well in removing suspended solids and particularly heavy metals from runoff. For example, Legret showed that suspended solids and lead can be reduced by PPS up to 64% and 79%, respectively. Kellems showed that enhanced filtration using organic media was an effective alternative to chemical precipitation for the treatment of storm water. Filtration through a specific adsorbent organic medium can remove about 95% of dissolved copper and zinc.
In comparison to pavements made of asphalt, concentrations of zinc, copper and lead were significantly lower on permeable structures. Lead concentrations were in fact undetectable. A PPS should regularly be kept clean to prevent clogging. Generally, PPS are efficient in trapping dissolved heavy metals in surface runoff. However, not all pavers and joint fillings have the ability to trap dissolved heavy metals. Pavements with large joints for infiltration must have a suitable joint filling. Otherwise, metals will pass through them, and may subsequently enter groundwater resources. Particles usually accumulate in geotextiles and on pavement surfaces. Geotextiles usually separate micro pollutants such as cadmium, zinc and copper from the underlying soil, therefore preventing groundwater from becoming contaminated.
3.4. Microbiology
PPS are powerful in-situ bioreactors, which can reduce hydrocarbon contamination by 98.7%. Biodegradation in PPS is enhanced by bacteria and fungi. When inoculated with microorganisms, the protozoan population diversity within a PPS increases more rapidly than in a similar non-inoculated system. Pavements contain testate amoebae, ciliates, flagellates and gym amoebae. The understanding of microbial biodiversity helps to interpret biodegradation mechanisms. PPS have the capacity to degrade large quantities of clean motor oil. Bio-treat HD, a commercially available oil degrading microbial mixture, will not degrade oil any better that the local microbial biomass established within the pavement over a long period of time. However, the local microbial biomass can only achieve high degradation rates, if there is adequate supply of nutrients (i.e. nitrogen and phosphorous) in the feed. Monitoring of biofilm development through scanning electron microscopy has revealed that a PPS can obtain a high degree of biodiversity due to the development of complex microbial compositions.
The assessment of the microbiological water quality has been an important process in preventing waterborne diseases. The two most common alternate tests carried out are for coliforms and Escherichia coli, or faecal coliforms. Total coliforms, faecal coliforms, faecal streptococci, heterotrophs, fungi, Pseudomonas aeruginosa, Leptospira, salmonellae and viruses are often analysed in an attempt to determine the temporal distribution of bacterial pathogens and viruses in storm water runoff. However, findings usually show that it is not possible to accurately predict the time when peak microbial populations including human pathogens occur in runoff waters. 5. Recent research works.
5.1. Combined Geothermal Heating and Cooling effect 5.1.1. Introduction to Geothermal Heat Pumps.
The technology has many names: ground source heat pump (GSHP), ground coupled heat pump (GCHP), geo-thermal heat pump (GHP), geo-exchange (GX), Earth energy system. Geogeo-thermal heat pumps (GHP) or geo exchange systems are commonly used in North America, China, Japan and some European countries. Most GSHP use refrigerant to move unwanted energy (i.e. heat) out of buildings during summer and into them (if required) during winter. (Bose JE. (2005)) they use constant temperatures of surrounding grounds, which are lower than the corresponding air temperatures during warm seasons (heat sinks) and higher during cold seasons (heat sources). For ground connections, plastic pipes are installed within the soil. Various applications such as horizontal, vertical, Looped or submerged designs can be used. The main thermal carrier within coils is a mixture of water and a de- icing agent. The length and width of the loops is determined by ground conductivity abilities. The most important variables are type of soil, geology and area of available land for such installations.
5.1.3. Integration of PPS with Geothermal Heat Pumps
Environment not only prevents and reduces the risk of flooding and pollution of watercourses, but also reduces energy costs by the application of a green energy source (earth energy) which adds many other environmental benefits (Scholz and Graboweicki, 2008). Permeable pavement engineering is an effective and simple method of providing structural pavements whilst allowing storm water to infiltrate freely through the surfaces for temporary storage, storm attenuation, dispersal and reuse. Permeable pavement systems (PPS) are a sustainable urban drainage system (SUDS) whereby water from urban runoff can be treated by filtration and sedimentation for recycling, harvesting or reuse purposes. Geothermal Heat Pumps (GHPs) also referred to as ground source heat pumps are receiving increasing interest because of its potential to reduce primary energy consumption, reduce emissions of greenhouse gases and thus reduce the effects of climate change (Tota-Maharaj et al., 2009).
Figure. 20. Cross section of PPS Figure. 19. Process of heat transfer in GHP
5.1.2. Health risks associated with permeable pavement systems.
Various applications of PPS have been tested in the UK. The main pollutant used was mineral engine oils containing hydrocarbons. Furthermore, a specific geotextile incorporating polymer beads was designed to release nutrients for better microbial community growth and more efficient hydrocarbon removal. Spicer GE et al. (2006) this ‘self- fertilising’ geotextile demonstrates that nutrient sources can be incorporated into a polymer composite to influence positively microbial growth within PPS. A different approach has been taken at ‘The University of Edinburgh’. The research team had decided to use gully pot liquor mixed with tap water and animal faeces in some experimental rigs as the main pollutants. Such mixtures would mimic the most extreme conditions that may occur in practice. Gully pot liquor provides all possible pollutants available naturally. A gully pot is a biochemical reactor where pollutants are released after acidic dissolution and sediment maturation. Also various microbial degradation processes take place in the gully pot chamber. Morrison GM et al. (1995) because gully pots operate under various regimes (e.g. wet or dry), concentrations of dissolved oxygen can drop rapidly during anaerobic conditions. Butler D et al. (1995) Because of their potential pathogenic nature, animal faeces (e.g. dog droppings) are not commonly used pollutants in academic SUDS research. Nevertheless, there are serious health concerns associated with PPS water (particularly if contaminated with faecal matter), which could potentially be recycled within buildings for toilet flushing and other applications as discussed by the authors previously (Grabowiecki P et al. (2006)). Both PPS and GSHP are commercially available applications, but were never used as combined systems in a research project. A combined
system has the potential to capture, detain and treat runoff, and to either cool or heat a nearby building at the same time. The sub-base is only heated passively during the summer when hot air with in an adjacent building needs to be cooled down by transferring access heat to below the permeable pavement. The heat is in fact a waste product that has the additional benefit of enhancing biodegradation within the sub-base.
The physical attenuation of storm water pollutants by permeable pavements with varying designs of geotextile membranes. The research would give an indication as to the effects of contaminants present in urban runoff and the possibility of biodegradation by anaerobic processes occurs. One of the guiding principles of SUDS is centred on mitigating adverse effects of urban storm water runoff such as increased urban flooding and deteriorating receiving water quality. SUDS such as permeable pavements are commonly perceived as an effective source control measure to reduce storm water flows and pollution loads. However, there have only been limited studies aimed specifically at quantifying the effectiveness of utilising permeable pavements as a source control measure. The recycling and reuse of rainwater, using permeable paving as the reservoir for storage, shows great potential in the reduction of mains water use for low grade uses. Water for toilet flushing, landscaping and car washing can be stored in the pavement structure and pumped out for reuse.
5.2.1. Rainwater harvesting after storage in permeable pavements
Permeable pavements built to the Hanson Form pave specification have an average storage capacity of around 1m3 per 10m2 of paving with an excavation depth of 500 mm. This storage capacity is easily realised by the
provision of a lining system, usually based on a Visqueen-type material. Once the build is complete, accessories such as electric pumps can be added in order to move the water into the house for WC flushing or for external use such as landscaping. The onsite use of rainwater for non-potable purposes in both domestic and industrial settings shows great promise in reducing the need for highly purified mains water. In a domestic setting at certain times of the year, over 50 % of water could be supplied by the use of rainwater, certainly when feeding sprinkler systems in dry weather. The use of a permeable pavement as the storage element in rainwater harvesting makes efficient use of a large potential storage volume, adheres to SUDS principles and value engineering good practice, realising significant savings for the end user.
5.2.2. Contamination of stored rainwater
When rainwater falls onto an urban receiving surface, whether that surface is a roof, road, pavement or car, it usually becomes contaminated relative to the water quality of the original rainfall. This is because rainfall is relatively free of contaminants except for materials like pollen, microbial spores, very fine dust and sometimes dissolved gases such as SO2 and NO2. After falling and coming into contact with surfaces, non-permeable urban surfaces tend to have any pollutants on them scoured off by the rainfall and moved, sometimes quite a large distance from their origin (Charlesworth et al, 2003). If rainwater is to be harvested for later use then there are many measures available to remove the larger contaminating waterborne fractions; these include filters, screens or grit traps. These measures are very effective in preventing blockages of pipework, but do not remove most of the smaller particles which may find their way into the storage reservoir. These smaller sized fractions include microorganisms typically between 2 and 20 micrometres in size.
5.2.3. Microbiological contamination of harvested rainwater
Any microbiological contamination in water harvesting schemes would come primarily from animal wastes from cats, dogs, rodents or birds. The unpredictability of any such contamination episodes is one of the reasons why such phenomena are difficult to study. It is clear that at times the number of potentially harmful microbes in urban water, e.g. faecal coliforms, is high (Ferguson et al, 1996: Butler and Davies, 2000). Potentially there are a number of possible contaminating microbial types in harvested rainwater, of different taxonomic descriptions (Evans et al, 2006; Grabiowiecki et al, 2008) and which may increase in number under a variety of different conditions. It should be noted that most of the organisms would come from animal faeces and this represents the most likely contamination of the stored water. Since there is no definitive data on the microbiological safety of rainwater harvesting systems, many harvesting systems have a microfilter which would remove most bacteria and protists from the system. Such filter systems are relatively susceptible to faults such as blockages if a large amount of suspended material is in the water, for example after intense rainfall. UV light sterilisation kits are also available that would remove the vast majority of micro-organism from the water, although these are relatively expensive to install and also require some maintenance. However, in general, a properly designed, installed, monitored and maintained harvesting system should provide excellent water saving benefits with little risk to the user.
Despite these safeguards, from an information gathering perspective and also to simulate a ‘worst case’ scenario, it was decided to apply two selected microbial contaminants from the above list to a simulated permeable paving system to determine the density of these microbes within the paving system after contact with the Aqua flow geotextile, Inbitex is known to very effectively filter out chemical and particulate pollutants from water (Newman et al, 2002). The interactions between the contaminants and the indigenous microbial population of the paving system were also of considerable interest to reduce the microbial effects on the urban runoff.
3. Innovations and Future Research
To date, the application of permeable pavement systems has been limited to roadways for vehicular travel. Ongoing and future research could potentially allow for new and innovative applications such as with airport runways. There is a myriad of different applications where human’s quest for development is hindered by environmental consequences. It is possible that with the use of permeable pavements these events may not be so catastrophic. Landslides are one example. Until recently, landslides were only thought to be associated with high
intensity rainstorms on steep inclines. However, recent studies have shown that the threshold of rainfall intensity versus duration for shallow sloped landslides to occur is lower than previous estimates (Guzzetti, E. et al. 2007). It was found that the rainfall intensity plays a more important role in increasing the chances for landslides to occur due to the sheer quantity of water draining over a short amount of time (Guzzetti, E. et al. 2007). These conditions can be exasperated by human development, which alters the drainage path of the rainwater, increasing the likelihood of a landslide (Ozdemir, A et al. 2008). This is where the permeable pavement design could come in to play. Virtually all the studies conducted on permeable pavement have noted its incredible hydrological properties; because the permeable pavement allows water to pass through it in to the soil it does not significantly alter the drainage path of the rainwater. This means that structures could potentially be built with a permeable foundation. So, they would have little impact on the surrounding environment. The research focuses also on improving the growth of microorganisms during artificial temperature fluctuations induced by the heat pump. Further research on the short and long-term effects of contaminants that remain in the PPS should be undertaken. The self-sustainability of these relatively new systems in comparison to traditional pavements requires further assessment. Moreover, the long-term impact of PPS on the environment is still unclear. Before all of this can be accomplished though more research has to be put into improving the lifespan as well as decreasing the costs of permeable pavement. Hopefully if these two negative aspects of permeable pavement can be eliminated these systems can be installed in more places around the world.
6. Conclusions
This paper looked at various studies conducted on permeable pavement systems and their current application. Also discussed about the detailed design of permeable interlocking concrete pavement in brief. Maintenance and water quality control aspects relevant to the practitioner were outlined for permeable pavement systems. The water quality aspects is highlighted. Recent innovations were highlighted and explained, and their potential for further research work was outlined. The recent innovations like development of a combined geothermal heating and cooling, water treatment and recycling pavement system is promising and it is detailed in cut short, future research works are outlined in brief.
These permeable pavement systems are changing the way human development interacts with the natural environment. Its application towards parking lots, highways and even airport runways are all improvements in terms of water quality, water quantity and safety.
7. References
[1]. North Carolina Department of Environment and Natural Resources (NCDENR). NCDENR Storm water BMP Manual. NCDENR. North Carolina, 2007.
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